1. Field of the Invention
The present invention relates generally to a Mach-Zehnder interferometer for an optical communication system. More particularly, the present invention is directed to a Mach-Zehnder interferometer, formed from segmented cane structures, which provides access to the constituent optical fibers during the fabrication process.
2. Technical Background
There is an emerging need for narrowband wavelength division multiplexing (WDM) couplers and filters. Such optical devices are needed in the 1550 nm window for modifying the gain spectrum of erbium fiber amplifiers. These devices will also be widely used in trunk lines as well as in fiber-to-the-subscriber communication architectures. When used in these applications, these optical devices will need to be environmentally stable, unaffected by large temperature changes, and very reliable.
There is also a need for both wavelength tunable optical devices and fixed wavelength optical devices. In an all optical network, for example, the optical device can be tuned at the receiver end in order to detect the desired incoming signal. In a second approach, tunable lasers are used to send a plurality of signals, and the desired signal is detected by employing a receiver having a fixed filter. The transmission system could also employ both fixed lasers and filters. The wavelength separation capabilities of the filters for these systems needs to be on the order of tens of nanometers to as small as less then one nanometer.
The Mach-Zehnder interferometer is known for its narrowband wavelength filtering capabilities. It has been proposed that filters having pass bands as narrow as 1 nm be formed by connecting two evanescent couplers with unequal optical path lengths between them. However, it is difficult to achieve reproducibility and environmental stability with this approach, since the connecting fibers are subject to external destabilizing conditions such as temperature changes and random bending forces.
One technique for stabilizing a Mach-Zehnder interferometer is disclosed in U.S. Pat. No. 5,295,205, which is incorporated herein by reference, and commonly owned by the assignee of the present invention. As taught by this patent, the Mach-Zehnder interferometer is formed by threading first and second dissimilar optical fibers through the central bore of a continuous glass tube. The tube is evacuated and heated to collapse it onto the fibers. The tube is further heated and stretched at two spaced locations to form two couplers that are joined by the two optical fibers.
While the resulting Mach-Zehnder structure does improve the environmental stability of the optical device, this structure is still subject to thermal instability. Additionally, this structure does not allow access to the optical fibers in a region between the two coupling regions, known as the phase shift region. In view of these limitations of known Mach-Zehnder interferometers, it is desirable to provide a structure for realizing a Mach-Zehnder device which is highly insensitive to large temperature variations, or athermalized, and also allows physical access to the optical fibers within the phase shift region of the interferometer.